Locus Problem (3)

﻿In the applet below,
O is the center of the circle shown.
Point D is a point that lies ON this circle.
Point A is a point that ALWAYS LIES OUTSIDEthe circle. (You can move it anywhere you'd like).
The pink line is the perpendicular bisector of the segment with endpoints A and D.
Drag point D around the circle a few times. What do you see? Describe in detail!
Feel free to alter the locations of A and the gray point (radius changer).
Then clear the trace and drag point D around again.
Why does this occur?